The NIP (Nostrum Improvement Proposal) defines a new message called `AUTH`, which can be used for authentication between relays and clients in the Nostrum network. When sent by a relay, it includes a challenge string that the client must sign and send back to complete the authentication process. The signed event is an ephemeral event not meant to be published or queried, and should have at least two tags, one for the relay URL and one for the challenge string as received from the relay. Relays MUST exclude `kind: 22242` events from being broadcasted to any client.

Link: https://github.com/nostr-protocol/nips/blob/master/42.md